Posterior MI Changes

The above ECG strips were taken from a 58 years old was admitted two days ago with history of chest pain that started about 30 minutes prior to admission to Emergency Department. (Click on each ECG strip to get a clearer, larger view.)

The ECF obviously show ST elevations in leads II, III and aVF, indicating that he has acute inferior wall MI.

However, leads V1 and V2 also show ST depression, and particularly in lead V2, an increase R/S ratio of >1 (previous ECG strip taken prior to this also demonstrated increased height of R but the ST depression was no so obvious as this one).

So, this patient also had a posterior wall infarct besides the inferior wall MI (inferoposterior wall MI). Isolated inferior MI can occur but more commonly inferior wall MI extends to the posterior wall, right ventricle as well as lateral MI.

He was given fibrinolytic therapy and admitted to CCU after completion of fibrinolytic in ED.

Posterior STEMI
The posterior wall of the LV is perfused by the posterior descending coronary artery, which is usually originates from the RCA (and occasionally from the left circumflex artery).
Therefore, posterior MI usually associated with inferior or lateral wall MI

Posterior MI
Standard 12 lead ECG does not directly image the posterior wall. Therefore, identifying reciprocal changes in leads V1 & V2 (leads that are 180 degrees away from posterior region) help establish diagnosis of posterior MI

ECG changes of Posterior MI
1. ST depression V1, V2 (or V3)

2. R:S ratio ≥ 1
















Enlarged view of lead V2 with posterior MI changes.

Note:
ST depression in V2 may also be due to septal ischemia; but in this case R:S ratio less than 1 and R wave width less than 0.03 s

* For posterior MI R:S ratio is more reliable indicator of posterior MI compared to ST depression

However, increased R:S ratio ≥ 1 in V1 or V2 maybe due to other causes as well like:
a. RBBB
b. RVH
c. in young children and adolescents, can be a normal variant

To detect posterior MI, posterior leads V7 – V9 may also be used.

Lead V7 – posterior axillary line
Lead V8 – inferior angle of scapula
Lead V9 – left paraspinal border


Reference:
Chan TC, Brady WJ, Harrigan RA et al. ECG in Emergency Medicine and Acute Care. 1st ed. Pennsylvania: Elsevier Mosby; 2005.


Good web resources on posterior MI
1. For another clear, good picture of ECG showing posterior MI, click here
2. For a concise description on ECG changes of posterior MI, click here

Prognostic Value of ST Elevations in Myocardial Infarction

Anterior location of STE (anterior AMI) compared with inferior or lateral, correlates with much worse prognosis and greater benefit of reperfusion therapy

Patients with anterior infarction demonstrated a lower left ventricular ejection fraction on admission and after 10 days than did patients with inferior infarction, even after adjustment for infarct size, as well as a higher incidence of congestive heart failure and cumulative cardiac mortality.

Reference:
1. Chan TC, Brady WJ, Harrigan RA et al. ECG in Emergency Medicine and Acute Care. 1st ed. Pennsylvania: Elsevier Mosby; 2005.

2. Stone PH, Raabe DS, Jaffe AS et al. Prognostic significance of location and type of myocardial infarction: independent adverse outcome associated with anterior location. J Am Coll Cardiol 1988; 11 (3):453-63.

ST Score

Greater height of ST segments and greater number of leads involved correlate with higher mortality and greater benefits from reperfusion therapy.

ST Score refers to the total millimeters of STE in all leads combined. High ST Score correlates with higher mortality

Measured at J point, a score greater than 12 to 13 mm for anterior AMI and greater than 6 – 9 mm for inferior AMI correlates with high risk of early complications

For example, in a study by Gwechenberger et al, a cutoff value of 13 mm for anterior MI and 9 mm for inferior MI stratifies patients into high- and low-risk subgroups for the development of acute congestive heart failure and severe rhythm disturbances during the first 24 hours of hospitalization.


References:
1. Chan TC, Brady WJ, Harrigan RA et al. ECG in Emergency Medicine and Acute Care. 1st ed. Pennsylvania: Elsevier Mosby; 2005.

2. Gwechenberger M, Schreiber W, Kittler H et al. Prediction of early complications in patients with acute myocardial infarction by calculation of the ST score. Ann Emerg Med 1997; 30 (5):563-70.